Music from the TV and the ATV is always stereo on the soundbar only. Music from a Sonos source plays through all speakers, especially when te surrounds are set to full.

I haven’t tried it myself, but the above doesn’t match up with what I see on other forums — people are constantly raving about the surround mixes they get from Apple Music on their Sonos theater systems and say it only works through Apple TV.

It’s a question of how the signal comes from the TV. In general, most music apps send a stereo signal. If you’re using an external device that connects to your TV with an HDMI cable, that device can send a more complex signal, which is why you see theses claims from AppleTV folks. They’re using the applications on their device to send either a Dolby Digital or Atmos signal to the TV set, which the TV passes as a complex audio signal without video to the Sonos. If you were to probe in to those events, everyone would say they’re using the TV input on Sonos, and not normal ‘streaming’ from Sonos, which has a different path and  is treated differently in the Sonos software.